1. Field of the Invention
The present invention relates to an image display apparatus. More particularly, the present invention relates to, for example, an image display apparatus such as a HMD (head-mounted display) or HUD (head-up display) that operates by projecting a two-dimensional image formed on a liquid crystal display (LCD) onto an observer's eye on a see-through basis by using a holographic optical element (HOE).
2. Description of the Prior Art
Conventionally, various see-through-type image display apparatus have been proposed that present a projected image in a form superimposed on an image of the outside world by the use of a holographic optical element (U.S. Pat. No. 5,475,512, and Japanese Patent Applications Laid-Open Nos. 2001-4956, 2000-122589, H4-118620, and others).
However, a holographic optical element used as a combiner in an eyepiece optical system has a narrow range of angle selectivity, and therefore, in a conventional see-through-type image display apparatus, the observation pupil is small, which makes the displayed image difficult to observe. Giving the holographic optical element a wider range of angle selectivity results in giving it a wider range of wavelength selectivity, permitting light of wavelengths unnecessary for image display to be diffracted so as to reach the observer's pupil. Thus, the displayed image suffers from bleeding of colors comparable with chromatic aberration.